A number of commercial alloys derive their high temperature strength from fine dispersions of a second phase material. In creep-resistant ferritic materials, these dispersions are characteristically carbides. In high nickel superalloys, the dispersed phase is often an intermetallic compound such as Ni.sub.3 (Al,Ti). The thermal stability of the dispersed phase is an important factor, particularly in determining elevated temperature creep resistance. The term "Laves phase" refers to a hard intermetallic compound that is comparatively stable at high temperature. A dispersion of such a stable intermetallic compound in an iron matrix is thought to be more resistant to coarsening and softening at elevated temperatures than a carbide dispersion.
The concept of using intermetallic compounds such as Laves phases, rather than carbides, for increasing the high temperature strengths of alloys is not new, but optimum high temperature creep strengths have not been realized in oxidation resistant alloys that are fabricable by conventional techniques.
A fine dispersion of intermetallic compounds, such as Laves phases, can be precipitated in the matrix of an iron-based alloy by a suitable heat treatment. The alloy is solution heat-treated at a temperature high enough to achieve solution of second phase precipitates in the iron-rich matrix. The alloy is cooled at a rate which is sufficiently rapid to retain a single-phased metastable structure, and then it is aged at a lower intermediate temperature to precipitate the second phase as a fine dispersoid. This fine dispersoid of second phase hardens and strengthens the matrix. Overageing occurs when precipitation is complete, a maximum hardness is reached, and further elevated temperature exposure results in a coarsening of the second-phase precipitates and decreasing hardness and strength. The heat treatment is known as precipitation hardening, and both the magnitude of hardening achieved and the stability of the hardening response in terms of overageing are reflected in the elevated temperature strength of a precipitation hardened alloy.
Precipitation of Laves phases from supersaturated ferritic iron (.alpha.-iron) has been the subject of investigation for decades and the hardness variation with time at a given ageing temperature for numerous binary systems has been determined. In a comparatively recent study, Hornobogen, E., "Precipitation from Iron-Base Alloys", G. R. Speich and J. B. Clark, Eds., vol. 23, p. 31, Gorden and Breach Science Publ., N.Y., (1965), the binary systems Fe--Be, Fe--Ti, and Fe--W were studied. In another study of interest, Speich, G.R., Trans. Met. Soc. AIME, vol. 224, p. 850 (1962), Fe--Cb and Fe--Ti systems were considered. It is noted that secondary additions, in particular chromium, have been added to binary systems but the changes in the precipitation hardening characteristics were not usually significant and in some cases overageing and softening were accelerated. Reference is made to Vowles, M.D. J., and West, D. R. F., J.I.S.I., p. 147 (1973) for a discussion of this approach. The most recent literature data on Laves phase strengthening of ferritic iron are reported by the Center for the Design of Alloys, University of California, Berkeley, California. Reference is also made to Zackay, V. F., F. R. Parker, and D. Bhandarker, John Dom Memorial Symposium, AIME, Cleveland, Ohio, (Oct. 1972); Bhat, M.S., M.S. Thesis, Univ. of California, Berkeley, LBL2277, (April 1974); Jones, R. H., V. F. Zackay, and E. R. Parker, Met. Trans., vol. 3, p. 2835, (Nov. 1972); Zackay, V. F., et al., Materials Science and Engineering 16, p. 201, (1974); and Bhandarkar, M.D., et. al., Met. Trans., vol 6A, p. 1281, (June 1975). These studies are representative of the current state of the art and concern the precipitation of Laves phases from super saturated .alpha.-iron in the binary systems Fe--Be, Fe--Cb, Fe--Ti, Fe--W, Fe--Mo, and Fe--Ta.
The principal disadvantage of the prior art has been that with low alloying additions, stable hardening and strengthening responses have not been achieved. The lack of stability has been reflected in stress-rupture strengths lower than those achieved by some alloys strengthened by carbides, as is demonstrated in the current state of the art studies referred to above. Low alloying additions to achieve hardening and strengthening are desirable to allow for further additions, such as chromium and aluminum, to provide oxidation resistance without substantial reductions in fabricability. In most of the studies referred to, the data are limited to age hardening characteristics and elevated temperature creep data were not studied. As noted, in cases where elevated temperatures were studied, the elevated temperature strengths were low and for example, in one of the studies referred to (see the Bhandarkar et al and Zackay et al references), involving an iron-based alloy strengthened by the Laves phase Fe.sub.2 Ta, the 1,100.degree. F (593.degree. C) stress rupture strengths were below that of 422 stainless steel, and above this temperature, the strengths declined rapidly below useful values.